1. Field of the Invention
The present invention relates to a transport medium driving device, a transport medium driving method, and a program product for driving a sheet-shaped transport medium, and an image forming apparatus.
2. Description of the Related Art
In recent years, there is an increasing demand for a production printing system that provides various kinds of high quality images in a small lot size. In the electrophotographic image forming apparatus used in the production printing system, an electrostatic latent image is formed on a photosensitive element by optical writing and is developed into a toner image. The toner image is transferred onto a sheet and is then fixed by, for example, heat or pressure. In this way, an image is formed on the sheet.
In some of the typical full color image forming apparatuses, the toner image is transferred onto an intermediate transfer body, such as an intermediate transfer belt or an intermediate transfer drum, and then a color image is formed. That is, an operation of transferring the toner image onto the intermediate transfer body (i.e. the primary transfer) is performed on each color to superimpose the toner images of a plurality of colors on the intermediate transfer body. Then, a color toner image is transferred from the intermediate transfer body to a sheet (i.e. the secondary transfer). Then, the color toner image-on the sheet is fixed. In this way, a color image is obtained.
In the image forming apparatus that forms a full color image, a secondary transfer unit that transfers the toner image on the intermediate transfer body to the sheet includes a sheet transport unit. In the image forming apparatus, during the transfer of the toner image to the sheet, if there is a difference between the time when the toner image reaches the transfer unit and the time when the sheet reaches the transfer unit, a deviation occurs in the image formed on the sheet, which causes a reduction in image quality.
The deviation of the image during transfer occurs due to various factors. For example, the image deviation occurs due to a slip between the roller of the sheet transport unit and the sheet or a slip between the intermediate transfer body and a roller for driving the intermediate transfer body. In addition, the image deviation occurs due to, for example, a variation in the length of a sheet transport path caused by the deformation of a component, which occurs due to a change in temperature and humidity or a variation over time, and a variation in the sheet transport speed or the amount of transport due to a change in the diameter of the sheet transport roller. Further, the image deviation occurs due to, for example, an error in the detection system that detects the position or speed of the sheet.
Japanese Patent No. 3978837 discloses an image forming apparatus that includes a sensor which detects the position of a sheet on an image carrier and a sensor which detects the time when the sheet passes and controls the rotational speed of a roller of a sheet transport unit on the basis of the detection result. According to the technique disclosed in Japanese Patent No. 3978837, the image and the sheet can timely reach the transfer position of the image, without being affected by the expansion and contraction or slip of an image carrier belt.
In Japanese Patent No. 3978837, the speed of the sheet is changed by a predetermined speed profile such that the image and the sheet timely reach the transfer position. In the method disclosed in Japanese Patent No. 3978837, the image and the sheet can timely reach the transfer position. However, after the operation of making the image and the sheet timely reach the transfer position, a process of determining whether or not the operation of a sheet transport control system including the sheet is converged is not performed. When the sheet enters the secondary transfer unit without convergence of the operation of the sheet transport control system, that is, without removing the positional deviation or vibration of the sheet transport control system, after the timing of the image and the sheet is adjusted, there is a concern that, for example, density irregularity, image deviation, or a magnification error will occur, which results in a reduction in image quality.
In order to solve the problems, it is considered that a transport mechanism is driven without being vibrated in the sheet transport control system. However, in this case, the gradient of the speed profile is reduced and it takes a long time to make the image and the sheet timely reach the transfer position, which results in a low printing speed. In addition, the transport distance of the sheet needs to increase, which results in an increase in the size of the apparatus.